Audio amplifiers have been manufactured using class-A and class A-B configurations since the earliest days of radio. These amplifiers are the simplest to implement and have been manufactured using vacuum tubes and transistors. These class A and class A-B amplifiers, while simple and easy to manufacture, are inefficient. Their output stages operate at about 40% efficiency. resulting in heat generation. This means that for each 100 watts of input power, they only put out about 40W.
The concept of class-D amplifiers (often referred to as switching amplifiers) has been known for many decades. and class-D amplifiers have been used in control systems, where relatively low carrier frequencies are acceptable, since the 1960s. In the mid 1970s one of the co-inventors herein developed the first commercially successful class-D analog input audio amplifier. Class-D has the advantage of high efficiency, cooler operation and, ultimately, improved sound quality. Disadvantages have been the increased complexity and RF emissions. Implementing part of the class-D amplifier in an integrated circuit form and housing the amplifier in a sheet metal box has reduced the complexity and RF emissions, respectively.
Class-D amplifiers with analog inputs have been in manufacture for many years. During the late 1990s there has, in fact, been surge of activity in analog-input audio class-D amplifiers. The use of less energy and the generation of less heat are important considerations in multi-channel home theater amplifiers, for example, where five or more audio channels are typically found.
Audio sources are now migrating towards digital; the phonograph record has been almost completely replaced by the compact disk (hereinafter, xe2x80x9cCDxe2x80x9d) and digital audio tape (hereinafter, xe2x80x9cDATxe2x80x9d), and video tape is being replaced by the digital video disk (hereinafter, xe2x80x9cDVDxe2x80x9d). TV is becoming digital with the advent of direct broadcast TV. Computer audio is digital by nature.
This invention is accordingly directed to an amplifier that can accept the digital output of a digital source (e.g., CD, DVD, DAT) without digital/analog conversion. Audio amplifiers which can directly accept the digital output of CDs, DATs, and DVDs, as well as future formats of future media, are very desirable because they eliminate the need to have a digital-to-analog converter and thereby offer improved sound quality.
It is a further object of this invention to provide such an amplifier having a preferred design that can be implemented in one low-cost digital integrated circuit.
The invention herein is a digital class-D audio amplifier which accepts digital audio inputs and amplifies them without the use of a D/A converter. Briefly, the digital audio input is processed to create an error value of the audio input""s relationship with the previous audio input. The error value is used to pulse-width modulate a carrier signal, and the digitally modulated carrier is low pass filtered so that the amplifier output comprises an analog voltage capable of driving a speaker.
These and other features of the invention will become apparent from the following description of the preferred embodiment, of which the drawing is a part.